1. Field of the Invention
The present invention relates to a display device and a driving method thereof. In particular, the present invention relates to a display device to which an area gray scale method is applied and a driving method thereof.
2. Description of the Related Art
In recent years, a so-called self-luminous type display device having a pixel that is formed of a light-emitting element such as a light-emitting diode (LED) has been attracting attention. As a light-emitting element used for such a self-luminous type display device, an organic light-emitting diode (OLED) (also called an organic EL element, an electro luminescence: EL element, and the like) has been drawing attention and used for an EL display (for example, an organic EL display or the like). Since a light-emitting element such as an OLED is a self-luminous type, it has advantages such as higher visibility of pixels than that of a liquid crystal display, and fast response without requiring a backlight. The luminance of a light-emitting element is controlled by a current value flowing through the light-emitting element.
As a driving method for controlling a light-emitting gray scale of such a display device, there are a digital gray scale method and an analog gray scale method. In accordance with the digital gray scale method, a light-emitting element is turned on/off in a digital manner to express a gray scale. Meanwhile, the analog gray scale method includes a method for controlling the light-emitting intensity of a light-emitting element in an analog manner and a method for controlling the light-emitting time of a light-emitting element in an analog manner.
In the case of the digital gray scale method, there are only two states: a light-emitting state and a non-light-emitting state. Therefore, only two gray scales can be expressed if nothing is done. Accordingly, another method is used in combination to achieve multiple gray scales. An area gray scale method and a time gray scale method are often used as a method for multiple gray scales.
The area gray scale method is a method for expressing a gray scale by controlling an area of a lighting portion. In other words, gray scale display is performed by dividing one pixel into a plurality of sub-pixels and controlling the number or area of lighting sub-pixels (for example, see Reference 1: Japanese Patent Application Laid-Open No. H11-73158 and Reference 2: Japanese Patent Application Laid-Open No. 2001-125526). The number of the sub-pixels cannot be increased; therefore, it is difficult to realize high definition and multiple gray scales. This can be given as a disadvantage of the area gray scale method.
The time gray scale method is a method for expressing a gray scale by controlling the length of a light-emitting period or the frequency of light emission. In other words, one frame is divided into a plurality of sub-frames, each of which is weighted with respect to the number of light emissions and a light-emitting period, and then the total weight (the sum of the frequency of light emission and the sum of the light-emitting time) is differentiated for each gray scale, thereby expressing a gray scale. It is known that display failure such as a pseudo contour (or a false contour) may occur when such a time gray scale method is used and a measures against the failure is considered (for example, see Reference 3: Patent Publication No. 2903984, Reference 4: Patent Publication No. 3075335, Reference 5: Patent Publication No. 2639311, Reference 6: Patent Publication No. 3322809, Reference 7: Japanese Patent Application Laid-Open No. H10-307561, Reference 8: Patent Publication No. 3585369, and Reference 9: Patent Publication No. 3489884).
Although various methods for reducing pseudo contour have been conventionally suggested, a sufficient effect for reducing pseudo contour has not been obtained yet and further improvement has been urged.
For example, it is found that a pseudo contour cannot always be prevented by focusing attention on certain two pixels in accordance with a halftone display method in Reference 4. As a specific example, it is assumed that a gray scale level of 127 is expressed in a pixel A and a gray scale level of 128 is expressed in a pixel B adjacent to the pixel A. A light-emitting state and a non-light-emitting state in each sub-frame of this case are shown in FIGS. 64A to 64B. For example, FIG. 64A shows a case where one sees only the pixel A or the pixel B without turning one's eyes away. A pseudo contour is not caused in this case. This is because the sum is taken with respect to the luminance of the place where one's eyes pass; therefore, one's eyes sense the luminance. Thus, eyes sense the gray scale level of the pixel A to be 127 (=1+2+4+8+16+32+32+32), and eyes sense the gray scale level of the pixel B to be 128 (=32+32+32+32). In other words, eyes sense an accurate gray scale.
On the other hand, FIG. 64B shows a case where eyes move from the pixel A to the pixel B or from the pixel B to the pixel A. In this case, eyes sometimes sense the gray scale level to be 96 (=32+32+32), and sometimes sense the gray scale level to be 159 (=1+2+4+8+16+32+32+32+32) in accordance with eyes' movement. Although the eyes are expected to sense the gray scale levels to be 127 and 128, they sense the gray scale levels to be 96 or 159. Consequently, a pseudo contour is caused.
FIGS. 64A to 64B show a case of 8-bit gray scales (256 gray scales). Next, FIG. 65 shows a case of 4-bit gray scales (16 gray scales). In this case also, eyes sometimes sense the gray scale level to be 4 (=4), and sometimes sense the gray scale level to be 11 (=1+2+4+4) in accordance with eyes' movement. Although the eyes are expected to sense gray scale levels to be 7 and 8, they sense the gray scale levels to be 4 or 11. Consequently, a pseudo contour is caused.